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I am trying to create a pen tool using mouse listeners:

public void mouseDragged(MouseEvent e) {
            imageL.setCoordinates(originalPos, e.getPoint());
            imageL.repaint();
            originalPos = e.getPoint();
        }

The paint function in the JLabel (imageL) receives two sets of points that allow drawing a line based upon the mouse drag. The only issue is that every time the drag is performed, the new layer does not contain the line drawn from the previous mouse drag. The paint function of the JLabel is as follows:

public void paint(Graphics g) {
    super.paint(g); 
    Graphics2D g2d = (Graphics2D)g;
    g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
            RenderingHints.VALUE_ANTIALIAS_ON);
    g2d.setColor(drawingColour);
    g2d.drawLine(originCors.x,originCors.y,endCors.x,endCors.y);
}

So essentially my question is: How do I "add" the new line to the current layer?

any help would be great, Thanks in advance

Michael Ole
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2 Answers2

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You want to draw in a JComponent's (and JLabel extends from JComponent) paintComponent method, not the paint method unless you plan to handle all the painting of the component's borders and children. Next, you may wish to have your drawing component hold an ArrayList of Point and add Points to this array list in the mouseDragged method. Then the paintComponent method can iterate through the list and paint all the lines. If all you want to do is paint one line, then have the painting JLabel hold both points in a class field.

Hovercraft Full Of Eels
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    +1 for `paintComponent()`. See also this [answer](http://stackoverflow.com/questions/5797862/draw-a-line-in-a-jpanel-with-button-click-in-java/5797965#5797965). – trashgod May 07 '11 at 01:08
  • i ll go through the example right now, thanks for your help guys – Michael Ole May 07 '11 at 01:18
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Michael,

For a start, in swing (a JLabel is a swing component, as apposed to the older AWT library) the recommended practice is to override the paintComponent method (not the "paint" method). See The Java Tutorials: Custom Painting for the hows & whys of it.

If you want to custom-paint a-list-of-lines then you're going to have to do just that... not just the "new" one. One way around this is to "update" an image with each new line, and then custom-paint that... this is slightly quicker as you only "paint" the each line once, at the cost of using more memory (to maintain the "backgroun image")... but this technique lends itself to "double buffering", which avoids "that flickering" you get when you draw directly to the screen. See The Java Tutorials: Working with Images for the details. It's actually pretty straight forward.

I suggest you google for "java double buffering example" (avoid Rose India, it's full of crap).

Cheers. Keith.

corlettk
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